"fully voiced 3d original game" is a whole different scale, budget, and cost risk than "colloseum" side game, which was barebones, but still the highest selling GC rpg I believe.
If you mean the colloseum game, still, it's not as needed since pokemon battles happen online and are in 3D now. And definitely wouldn't be a launch game in any case.
I stopped playing pkmn after...I guess ruby/sapphire, but in the midst of talking about a huge franchise with a huge decline (that still has me confused as someone that doesn't follow Halo. I mean wtf happened?), you can't deny TPC knows that they're doing...business wise.